1. Field of the Invention
This invention relates to a receiving apparatus and demodulating method used in a Multi-Input Multi-Output (MIMO) system.
2. Description of the Related Art
A Multi-Input Multi-Output (MIMO) system is configured to receive a transmit signal which is transmitted with the same frequency at the same time from a plurality of transmitting apparatuses, by a receiving apparatus having a plurality of antennas. As methods of separating signals transmitted from the respective transmitting apparatuses from the receive signals, in this system, receiving schemes such as a spatial filtering and a sequential decision and decoding are known (for example, P. W. Wolniansky, G. J. Foschini, G. D. Golden, R. A. Valenzuela, “V-BLAST: An Architecture for Realizing Very High Data Rates Over the Rich-scattering Wireless Channel” invited paper, Proc. ISSSE-98, Pisa, Italy, Sep. 29, 1998).
In the MIMO system, signals are transmitted from a plurality of transmitting apparatuses after transmission sequences of the respective transmitting apparatuses are subjected to error-correcting-coding. In the receiving apparatus, the transmit signals are received with antennas whose number is equal to or greater than the number of the transmitting apparatuses, and propagation path H for each of the antennas is estimated on the basis of a pilot signal inserted into the signal received by each antenna.
The propagation path H thus estimated is expressed as a 2×2 matrix if, for example, two transmitting apparatuses exist and two receiving antennas exist. The spatial filtering is a method by which the transmitted signals are obtained by multiplying an inverse matrix of the propagation path H thus obtained by the receive signals and then subjected to error-correcting-coding, to obtain the transmission sequences.
The sequential decision and decoding is a method by which a replica signal is formed by multiplying one of the transmit signals obtained from the spatial filtering by the propagation path matrix, fed back to the receive signals before subjected to the spatial filtering, and subtracted from the receive signals, to improve the demodulation performance of the other transmit signal.
If propagation paths have a high correlation, however, the signals cannot be separated at high accuracy by the spatial filtering. In this case, the receiving performance cannot be improved even if the replica signal is fed back by the sequential decision and decoding.
In the prior art, the transmit signals of the respective transmitting apparatuses have been interleaved in different patterns to reduce the characteristic degradation resulting from spatial correlation (for example, Jpn. Pat. Appln. KOKAI No. 2003-32226 and Murakami et al. “Review of application of interleaving of iterative decoding using deletion of signal points in the MIMO system” Shingaku-Giho RCS 2004-8). However, since a large memory has been required for application of such interleaving, it has complicated configuration of the transmitting apparatuses and receiving apparatuses and prevented downsizing and low power consumption of the apparatuses.